1. Field of the Invention
The present invention relates generally to magneto-optical disk head assemblies, and more specifically to a magneto-optical disk head assembly having improved frequency response characteristics.
2. Description of the Related Art
In a conventional head assembly for magneto-optical recording disks, the light from a semiconductor laser is collimated into a parallel beam, in which a beam splitter is located, and is focused onto the surface of a magneto-optical disk. Light rays reflecting off the disk are collimated and reflected by the beam splitter off to a birefringent holographic grating, or diffracting plate having different diffraction patterns 1 through 4 as illustrated in FIG. 1 and whose optical axis is at 45.degree. to the polarization plane of the incident light. The light passing through the diffracting plate has three optical components, i.e., the undiffracted, zeroth-order component (ordinary light) and the plus first-order and minus first-order diffracted components (extraordinary rays), which impinge on light sensors 5, 6 and 7, respectively, arranged on a light detector such that the zeroth-order component forms a light spot 10 on sensor 5, the plus first-order component forming spots 11-14 on sensor 6, and the minus first-order diffracted component forming spots 15-18 on sensor 7. Specifically, the diffraction patterns 1-4 positionally correspond to light spots 11-14 on sensor 6, respectively, and further correspond to light spots 15-18 on sensor 7, respectively. The sensor 6 is subdivided into an upper sensor 6a illuminated with the spot 14, four adjoining intermediate sensors 6b-6e where the spots 11, 12 are formed, and a lower sensor 6f where the spot 13 is formed. By representing the electrical output signal from each light sensitive region as a voltage V with a subscript indicating the sensor from which it is generated, a focussing control signal, a tracking control signal and a readout signal are derived as follows: EQU Focussing control=(V.sub.6b +V.sub.6e)-(V.sub.6c +V.sub.6d) EQU Tracking control=V.sub.6a -V.sub.6f EQU Readout signal=V.sub.5 -(V.sub.6a +V.sub.6b +V.sub.6c +V.sub.6d +V.sub.6e +V.sub.6f +V.sub.7)
As illustrated, the focussing control signal is derived by the use of adders 20 and 21 and a subtractor 22 from sensors 6b, 6e, 6c and 6d, and the tracking control signal is derived by a subtractor 23 from sensors 6a and 6f. The readout signal is derived by adders 20, 21 and 24 and a subtractor 25 from all the sensors by subtracting the signal indicating the sum of plus first-order and minus first-order components from the zeroth-order component signal.
Since one input of subtractor 25 is derived exclusively from the plus-minus first-order diffracted light components and the other input of the subtractor is derived exclusively from the zeroth-order light component, these subtractor inputs exhibit different frequency response characteristics. As a result, the readout signal suffers frequency dependent effects arising from the inherent intensity variation of the source laser and the common-mode noise due to variations of the disk's reflectivity.